There's a fine line between sharing for the good of a community and spamming the community in self-promotion (intentional or not). And as a professional consultant, I try very hard to walk that fine line without crossing into spam territory.
The 4 warnings signs below highlight my views on what's spam and what's not.
1. Your post promotes a product or service
When starting a new thread of discussion with a new post, consider whether the content you're sharing mentions a specific paid app from the AppExchange or highlights the success of a specific consulting company. If so, chances are people are going to feel like you're spamming.
I do want to highlight a difference between posting new (unsolicited) instead of responding to a question or an explicitly expressed need (solicited). In the latter case, you could actually be helping by pointing the other person to a solution, paid or otherwise, that really addresses the need.
Consultant's note: Sometimes I come across a question on the community that seems like a lead for a new project at a prospective client. Personally I do still take time to answer the question in as much detail as I think is feasible, so that the poster has enough guidance to either implement the solution on his own or to contract a partner to help. Posting bluntly, "you need a partner to help" would be inappropriate.
2. You blindly cross-post your blog across multiple groups
With Chatter, it's super easy to share a single post across multiple groups, either by sharing (i.e., reposting) the original post or by mentioning other groups in a comment. If you're simply cross-posting without assessing and repackaging the content to make it relevant to each new audience, chances are you're spamming.
Consider for example a notification about an MVP Office Hours conference call, offered free of charge to the community. People share this information with their local user groups, and generally speaking no one complains about this being spam, because the resource is relevant and targeted to users, who are naturally members of user groups.
But is the same notification appropriate for an official group that's created specifically for a feature or different topic of interest, such as the Salesforce1 group or the Communities Implementation group? Now the cross-post is starting to smell like spam, unless additional context can be provided to explain that there will be a dedicated segment to mobile apps or Communities.
3. Your post has no clear relevance to the audience
Take a look at the charter or description of a group or any channel, and assess whether your post has relevance and value to add to that group. Obviously if the value is unclear or loosely associated, your post is probably spam. But I think it is possible to tweak or augment a post with additional information to make it relevant to a new audience.
Let's use Lightning Process Builder as an example in the context of Salesforce. A generic statement that adds unclear value could be, "Process Builder is the future of declarative automation on the Salesforce1 Platform." Posting this as-is to a Sales Effectiveness group is probably a bad idea. But adding additional context could make the post relevant, such as, "Here's an example of a process that intelligently populates the Next Step field with a recommended action based on field values on the opportunity and account."
4. You have no prior relationship with the audience
This one should be a no-brainer. Did you join a group or cross-post to a group just to share a piece of content? Even if you're doing it with the best of intentions, your action could be seen as spam. This is a human consideration, not necessarily a technicality.
Closing thoughts
Use common sense. None of the signs above are hard and fast rules. But instead, think of them as factors that comprise a "contributor score" similar to a Sender Score. Reputations are difficult to build and easy to ruin, so don't lie to yourself about what you're trying to do.
Treat your audience as real people, and imagine you were telling each person individually about whatever content you're about to share. If you think the vast majority of people will be appreciative, go for it. If not, you should probably err on the side of caution.
P.S. At the risk of being seen as promoting religion, I also want to share Matthew 18:15-17. This is 5 sentences well written about conflict resolution that applies to resolving differences of opinion about what is and isn't spam.
